This invention relates to a mass flow meter relates to a mass flow meter used in a device requiring a light flow meter of a gram order, and particularly relates to a mass flow meter suitable for the measurement of a mass flow rate of an artificial heart for an operation, a bedside type artificial heart, or an implantable artificial heart.
The measurement of the flow rate is indispensable in various kinds of technical fields, and many flow meters are conventionally proposed. In these flow meters, the mass flow meter can measure the flow rate converted in a standard state irrespective of using temperature and pressure of a measured fluid. Therefore, the mass flow meter is indispensable as a flow meter able to make a precise measurement. In such a mass flow meter, for example, a thermal type flow meter and a Coriolis type flow meter exist. However, in the thermal type flow meter, it is necessary to heat the fluid. Therefore, the thermal type flow meter is suitable for gas notable in a temperature rise, but is not suitable for a liquid of high density. Accordingly, the thermal type flow meter can be applied to only a liquid of a very small flow rate. Further, no fluid unable to be heated can be measured in the thermal type flow meter. Accordingly, it is particularly difficult to apply the thermal type flow meter to a liquid such as blood having high viscosity and unable to be heated. Further, in the Coriolis flow meter, problems exist in that a vibration mechanism device of a U-character tube is required, and is large-sized and price is high, and pressure loss is inevitable from the features of a shape, and it is difficult to clean the device when a U-character tube portion is blocked, etc. Accordingly, no Coriolis flow meter is suitable for the flow rate measurement of the fluid in which a component is changed by vibration.
On the other hand, in Japan, the organ transplant law is enforced, and heart transplantation from a person of brain death can be performed. However, donors are insufficient in the real situation. Therefore, a way for rescuing a left patient is only the artificial heart. The artificial heart is researched for a long time, and many clinical applications are reported. In the artificial heart, there are a ventricular assist device for inserting a natural heart in parallel without cutting the natural heart, and a total replacement artificial heart for cutting and connecting the natural heart. Conventionally, these artificial hearts are almost of an air driving type for arranging a controller in a bed site. However, in recent years, a ventricular assist device for an abdominal implantation, and electrically operated by using a battery attached to a belt or a rucksack is also developed. In present products, the artificial heart able to be remedied at home is used although it is limited to the artificial heart for a patient of a large physical constitution from a point of its size. In addition, a blood pump for an operation is comparatively used for a long period, and a similar mechanism is also used for circulatory assist device. Here, these are generally called the artificial heart.
When such an artificial heart is classified from the point of a pump type, two systems constructed by a pulsatile flow system and a continuous flow system generally exist. The pulsatile flow type is a system for sending-out the blood of a constant amount every one pulsatile output, and has the actual using results of a year unit in the ventricular assist device advanced in clinical application. The continuous flow type is a system for sending-out the blood at a constant pressure by a rotating mechanism. In this system, a sending-out amount is not directly relative to pump volume, and the system is easily made compact, and is promising for the implantable ventricular assist device. In reality, the natural heart is left, and for example, a ventricular assist device 32 is connected by a blood tube 33 with respect to the left atrium or the left ventricle of the natural heart 31 as shown in FIG. 3. A pulsatile flow is physiologically preferable. However, with respect to an influence of a non-pulsatile flow at a heart stop even in the worst case, it is reported that any physiological problems do not occur in some animal experiments.
The development of the continuous flow type artificial heart is advanced in Japan, and there are individual types such as a centrifugal type, an axial flow type, a rotating volume type, etc. in a continuous Flow type pump. In each type, pump characteristics can be characterized in the relation of pressure, the flow rate, electric power and a rotational speed, and it is necessary to measure the flow rate of the pump and control a driving rotational speed. A centrifugal artificial heart as shown in FIG. 4A is invented by the present inventors as a concrete example of such a pump, and is patented as Japanese patent No. 2807786 (JP-A-10-33664). In accordance with this artificial heart, as shown in FIG. 4, a centrifugal type impeller 42 is supported by two bearings 46-48 and 45-50. An impeller drive unit 51 is arranged in the lower portion of a casing 47. A magnet group 44 built in the impeller is rotated and operated through a partition wall 49 by rotating a magnet 53 within this impeller drive unit 51. Thus, blood is flowed-in from an inlet 54 formed in a casing upper portion, and can be discharged from an outlet 43 arranged around the lower portion of the casing. A structure adopting a drive unit of a direct drive system formed by replacing a movable portion 53 with an electromagnet group is also developed as a means for rotating the impeller by the above magnetic coupling.
Further, the present inventors, etc. have developed a pump as shown in FIG. 4B in addition to the above centrifugal artificial heart. In the pump shown in this figure, in an impeller section 62 having plural impellers 61 extending in a radial shape, its central portion is released and forms an inflow portion 63 of blood. When the impeller 61 is rotated and operated, the blood is sucked from a cylindrical flow inlet 65 arranged in an upper side casing 64, and is discharged from an outlet 66 arranged in the upper side casing 64. An impeller support member 67 is fixed to a lower portion of the impeller section 62, and a bearing member 68 is fixed to the inside of the impeller support member 67. A hydrodynamic bearing groove 71 for Lower side thrust having a spiral shape pattern of a pump-in type is formed on a lower end face 70 of the bearing member 68. A hydrodynamic bearing groove 73 for upper side thrust having a spiral shape pattern of a pump-out type is formed on an upper end face 72. A fixing shaft 77 fixed onto a lower side thrust receiving portion 76 fixed to a lower side casing 75 is projected and fixed to a cylindrical passage port portion 74 formed at the center of the bearing member 68. An upper side thrust receiving portion 78 is fixed and supported by a fixing member 79 in an upper end portion of the fixing shaft 77. The above lower side thrust receiving portion 76 is arranged so as to be opposed to the hydrodynamic bearing groove 71 for lower side thrust, and the above upper side thrust receiving portion 78 is arranged so as to be opposed to the hydrodynamic bearing groove 73 for upper side thrust. Further, a spiral groove 80 for generating pressure is formed in a lower outer circumference of the fixing shaft 77. Permanent magnets 81 are arranged at an equal interval in the outer circumference of the impeller support member 67. An electromagnet 82 is arranged in the outer circumference of the lower side casing 75 so as to be opposed to the above permanent magnet 81. A motor of a direct drive system is constructed by sequentially changing the polarity of this electromagnet 82 and conducting an electric current, and is set to an impeller drive unit 83.
With respect to the centrifugal artificial heart, patent literature 1 proposed by the present inventors, etc. exists as mentioned above. Further, density measuring portions are respectively arranged inside and outside a bending pipe. A density difference caused by centrifugal force of a liquid on the inside and the outside of the bending pipe is measured by these density measuring portions. The volume flow rate is then calculated by an arithmetic device on the basis of this density difference. Such a technique is disclosed in patent literature 2.
[Patent literature 1] JP-A-10-33664
[Patent literature 2] JP-A-9-79881